Dendritic cells (DCs) orchestrate either tolerance or immunity. At the heart of this important function lies phagocytosis, which allows DCs to sample the tissue microenvironment and deliver its constituents into endocytic compartments in a process called phagosome maturation. Two important outcomes of phagosome maturation are major histocompatibility complex (MHC) class II presentation and cross-presentation, both of which have important consequences on the activation of CD4 and CD8 T cells, respectively. Crosspresentation allows DCs to acquire antigen from infected or abnormal cells and safely orchestrate MHC class l-restricted responses. During phagocytosis, DCs establish self/non-self discrimination based on the differential engagement of Toll-like receptor (TLR) signaling pathways. We have shown that TLRs control phagosome maturation and one of its consequences, MHC class II presentation. Our current proposal is focused on addressing whether cross-presentation, which relies on the same internalization pathways necessary for MHC class II presentation, is also subject to regulatory control. Our main hypothesis is that cross-presentation is positively regulated by signals from TLRs at multiple levels. We base this hypothesis on our observations that i) phagosome maturation into processing endocytic compartments is induced by TLR signals, ii) MHC class II presentation is controlled by TLRs, and iii) phagosomes are autonomous organelles that individually regulate MHC class II presentation depending on the nature and origin of their cargo. Our long-term goal is to identify regulatory checkpoints that govern antigen presentation pathways within DCs, allowing cellular discrimination between self and non-self. Our specific aims are to: 1. Define the molecular regulation of cross-presentation by signals from TLRs. We will investigate whether TLRs control formation of peptide-MHC class I complexes from phagocytosed cargo, and whether this control is compartmentally restricted within DCs to phagosomes that contain TLR ligands. 2. Define the consequences of TLR control of MHC class II presentation on cross-presentation. We will determine whether TLRs control CDS T cell responses to cellular antigens by determining the availability of CD4 T cell help. This help is necessary for the proper activation and differentiation of CDS T cells. The immune system is tolerant to the body's own cells and tissues. This proposal aims to understand how signals that initiate immunity maintain the existing tolerance to self. The new knowledge we gain will further the design of therapeutic anti-viral and anti-tumor vaccines, and broaden our understanding of autoimmunity